The present invention relates to a method for deriving a standard 12-lead electrocardiogram effective for diagnosing ischemic heart disease, acute myocardial infarction, or the like, in which a minimum number of electrodes are attached to predetermined areas on the body surface of a living body. The present invention also relates to a monitoring apparatus using such a method.
Conventionally, when an electrocardiogram of a patient is detected, measured, and recorded in a hospital or a like facility, a total of ten electrodes are attached to the body surface of the patient; namely, six positions for chest leads, and four positions for limb leads. Six limb-lead waveforms (I, II, III, aVR, aVL, and aVF) of standard 12-lead waveforms and six chest-lead waveforms (V1, V2, V3, V4, V5, and V6) of the same are derived from electric potentials of the heart detected and measured by the ten electrodes by measuring means, such as an electrocardiograph.
The related-art electrocardiograph or the like can detect, measure, and record an electrocardiogram which is formed from standard 12-lead waveforms and allows appropriate diagnosis and treatment of a variety of heart diseases, through use of ten electrodes. Such a diagnosis and treatment using a plurality of electrodes is possible in a fully-equipped hospital, or the like, where a patient is maintained at rest. However, when at-home or emergency medical treatment is to be performed, no time is available for attaching a large number of electrodes to appropriate positions on the body surface of the living body, from the viewpoint of the status of the patient; moreover, difficulty is encountered in transmitting a large number of lead waveforms in the form of multi-channel signals. In addition, since only one channel (i.e., one lead) or a like number of channels of an electrocardiogram signal can be generally radio-transmitted, a heart disease is diagnosed with an electrocardiogram through use of a few number of electrodes (two to four electrodes).
Furthermore, the following configuration has been conventionally practiced as means for detecting and recording an electrocardiogram of standard 12-lead waveforms with a small number of electrodes. For example, four special positions (four electrodes of EASI) on the chest surface of a living body are used, and respective electrocardiographic waveforms thereof are lead. Once signals of the electrocardiographic waveforms have been converted into a vectorcardiogram with use of a fixed coefficient, the thus-converted vectorcardiogram is converted into a 12-lead electrocardiogram. The thus-obtained electrocardiogram is known as an EASI-lead electrocardiogram.
In the lead method of the EASI-lead electrocardiogram of the related art, an approximation to a standard 12-lead electrocardiogram can be attained to a certain degree. However, when leads from the four special positions on the chest surface of the living body are emplaced, appropriate attachment of the electrodes to respective specified positions encounters difficulty, since health care professionals, such as doctors and nurses, are not clinically accustomed to this attachment work, thereby posing a problem of variation arising in detection accuracy of the electrocardiogram. In addition, as described above, when arithmetic operation is performed to acquire the 12-lead electrocardiogram from the electrocardiographic signals derived from the electrodes, the signals must be converted twice (from EASI leads to a vectorcardiogram, and from the vectorcardiogram to a 12-lead electrocardiogram) through use of the fixed coefficient. Accordingly, in some cases variation arises in calculation accuracy. Furthermore, since none of the leads are actually measured values of the 12 leads, some doubts arise with regard to reliability.
General relationships among lead waveforms and measurement positions shown in FIGS. 4A and 4B, and potentials for obtaining a 12-lead electrocardiogram are as follows,
TABLE 1IvL − vRIIvF − vRIIIvF − vLaVRvR − (vL + vF)/2aVLvL − (vR + vF)/2aVFvF − (vL + vR)/2V1v1 − (vR + vL + vF)/3V2v2 − (vR + vL + vF)/3V3v3 − (vR + vL + vF)/3V4v4 − (vR + vL + vF)/3V5v5 − (vR + vL + vF)/3V6v6 − (vR + vL + vF)/3
Accordingly, in the lead method for the EASI lead electrocardiogram of the related art, positions to which the EASI electrodes are attached for measuring respective potentials are special and differ from those of the measurement positions of the lead waveforms of the case shown in Table 1. Therefore, accuracy in positioning to the specified positions in attachment of the electrodes poses considerable influences to a measurement result which is inconvenient, in that the attachment requires rich experience, and the like. In addition, even when a patent is resting in a fully equipped hospital, or the like, the number of the electrodes used for the standard 12-lead measurement is large. Accordingly, problems arise not only in terms of inconvenience for the patient, but also in terms of increased load on a health care professional who applies the electrodes.
From the above viewpoints, Japanese Patent Publication No. 2002-34943A proposes a method and an electrocardiograph for deriving a standard 12-lead electrocardiogram which enables appropriate diagnosis and treatment of a variety of heart diseases by making use of a lead system subset constituted of the minimum number of leads for obtaining a conventionally-known standard 12-lead electrocardiogram or an M-L leads (Mason-Likar leads) electrocardiogram; and filed a patent application therefor.
Specifically, this method utilizes, as a lead system subset Constituted of the minimum number of channels, for instance, leads I and II of limb leads in standard 12-lead system or ML leads, and leads V1, and V5 or V6 of chest leads which have been used for obtaining the standard 12-lead electrocardiogram. By virtue of the configuration, lead III and leads aVR, aVL, and aVF are calculated on the basis of intrinsic relationships among the leads shown in Table 1. The remaining chest leads V2, V3, V4, and V6 or V5 are calculated on the basis of relationships between the potential matrix, the lead vectors and the heart vectors.
In the case of limb leads, electrodes for deriving lead waveforms I and II are disposed at four positions constituted of the left and right arms (LA and RA), and left and right legs (LL and RL). In the case of ML leads, the electrodes are disposed at four positions constituted of parts below the left and right clavicles (LA and RA), and lower ends of the left and right anterior iliac spines or the left and right coastal arches (LL and RL). In this case, RL is caused to serve as a grounding electrode.
In addition, electrodes for deriving lead waveforms from two leads (V1, and V6 or V5) of chest leads are disposed at two positions constituted of the right margin of sternum in the fourth intercostal (V1), and a position on the left middle axillary line at the level of the fifth intercostal left midclavicular line (V6) or a position on the left anterior axillary line at the level of the fifth intercostal left midclavicular line (V5).
The lead system subset of the standard 12-lead electrocardiogram can be detected and measured; and the remaining leads of the standard 12-lead electrocardiogram can be calculated on the basis of the intrinsic relationships among the respective leads shown in Table 1.
A standard 12-lead electrocardiogram obtained as above utilizes the lead system subset of the related-art standard 12-lead electrocardiogram. Therefore, in attachment of the electrodes, positioning to the respective specified positions can be performed easily and without fail without requiring rich experience for the work. Hence, a highly-accurate standard 12-lead electrocardiogram can be derived, thereby enabling appropriate diagnosis and treatment of a variety of heart diseases.
As described above, when the patient complains of a symptom, such as chest pain or chest tightness, which is suspected to indicate angina pectoris or myocardial infarction, at the emergency treatment, an emergency medical staff, cardiovascular doctor, or the like, determines whether or not a 12-lead electrocardiogram includes an ischemic variation of the ST segment. If necessary, the patient is immediately transported to a cardiovascular-specialized hospital well-equipped with a CCU, and the like, which evidently leads to an increase in a survival rate of the patient.
Accordingly, desire has arisen for deriving a 12-lead electrocardiogram which is equivalent to a standard 12-lead electrocardiogram, in which leads of the well-known standard 12-lead electrocardiogram are simplified to the greatest possible extent, which has an accuracy sufficient to determine occurrence of angina pectoris and myocardial infarction, which is derived from a minimum number of electrodes, and, furthermore, in which attachment of the electrodes is facilitated.
On the other hand, under the method for deriving a standard 12-lead electrocardiogram disclosed in Japanese Patent Publication No. 2002-34943A, an instantaneous electromotive force vector (a heart vector) is obtained by making use of a subset of a lead system of a standard 12-lead electrocardiogram, and electrocardiographic potentials of unknown leads are calculated from the heart vector. According to this deriving method, limb lead electrodes are attached to positions of the ML-leads for use in an exercise stress testing electrocardiogram in place of two wrists and two ankles. More specifically, these electrode positions are located inward of the arm roots so as to be unaffected by arm movements during exercise, and are offset from an electrical lead line connecting the two arms and the heart. This offset can result in an error of lead vectors for calculation of an electrocardiogram derived from unknown leads.
In addition, as combinations of electrodes for deriving chest leads, leads V1 and V5 or leads V1 and V6 are selected. However, since electrical angles formed with these leads are close to 180 degrees, when an electrical angle formed with a heart vector and one of the pair of selected leads closes to the right angle, an electrical angle formed with the heart vector and the other one of the selected leads also closes to the right angle. In such a situation, since the lead potential closes to zero, probability of an error in calculation of the heart vector may be increased.